The present invention is directed to a process for making a composite elastic material and articles formed therefrom, having creep resistance, improved aesthetics, dimensional stability and inherent latency. The material is formed from an elastic fibrous web which is joined with at least one gatherable layer using the nip between an anvil calender roller and a point un-bonded calender roller having recessed areas on its surface.
Composite elastic materials and laminates thereof are known in the art as are methods for compression embossing fibrous webs. Composite elastic materials are gaining popularity for use especially in the areas of absorbent articles and disposable items because of the flexibility and conformability such materials provide the articles. The term xe2x80x9ccomposite elastic materialxe2x80x9d means a multicomponent or multilayer elastic material in which at least layer has an elastic component. As used herein, the term xe2x80x9claminatexe2x80x9d means a composite material made from two or more layers or webs of material which have been attached or bonded to one another. The term xe2x80x9cabsorbent articlesxe2x80x9d refers to devices which absorb and contain body exudates and, more specifically, refers to devices that are placed against or in proximity to the body of the wearer to absorb and contain the various exudates discharged from the body, and is intended to include diapers, training pants, absorbent underpants, incontinence products, medical applications such as surgical drapes, gowns and facemasks, articles of clothing or portions thereof including workwear and lab coats, and the like. Specific examples of such uses include, for instance, waistbands of diapers and training pants, side panels of training pants, and cuffs of surgical gowns. The term xe2x80x9cdisposablexe2x80x9d is used herein to describe absorbent articles not intended to be laundered or otherwise restored or reused as an absorbent article.
Research in this area tends to be focused on utilizing elastic materials in such absorbent articles to achieve a better fit for the wearer (more conformability to the user""s body), while continually searching for ways to improve the overall appearance and physical properties of the article. It is generally accepted by those in the field, that although elastic materials provide articles with better conformability, such articles generally do not have an attractive appearance or feel. The ultimate goal for such disposable items is to achieve a xe2x80x9ccloth-likexe2x80x9d appearance and feel, without compromising physical properties such as strength, elongation, and the like.
Another important property, known especially by those having skill in elastics, is a property known as inherent latency. As used herein, the term xe2x80x9cinherent latencyxe2x80x9d means the internal elasticity of a material, which is dormant until the material has been subjected to an activation process, for example, to elevated temperatures, as for instance, the temperature of the body of a wearer of the article. Furthermore, the process of converting these materials into such items as diapers, is usually conducted at elevated temperatures. Shrinkage of the material, due to the activation of the inherent latency, causes production problems as well. To quantify inherent latency, a test has been described in more detail below wherein percent shrinkage has been measured at an elevated temperature over a given period of time. When the temperature increases, e.g. to body temperature, the inherent latency is activated to improve the fit and conformability of the article. Controlling inherent latency has proved complex, however, because too much inherent latency may create too much elasticity, which may, for instance, cause over-tightening, resulting in xe2x80x9cred markingxe2x80x9d or irritation to the skin of the wearer. As an example, an article such as a diaper, having an appropriate amount of inherent latency, will neither tend to sag or droop while being worn (and subsequently saturated with body wastes), nor will it cause red marking. (As will be understood by those skilled in the art, there are many properties which contribute to red marking, such as adjustment of the basis weight of the material. For purposes of the present invention, the inherent latency is the property controlled to improve the materials). Such sagging or drooping is a result of too little inherent latency and has typically been quantified as stress relaxation and creep. The term xe2x80x9cstress relaxationxe2x80x9d is defined as the decreasing load required to hold a constant elongation over a period of time. The term xe2x80x9ccreepxe2x80x9d is defined as the loss of shape or dimension of an article due to some reversible and/or irreversible flow or structural breakdown under a constant load or force. There are two kinds of creep: (1) the time-dependent component, in which the shape changes because of the irreversible flow or structural breakdown under a constant load or force and does not recover when the force is removed; and (2) the time-independent component, wherein some of the shape recovers when the force is removed. Of course, one skilled in the art would understand that reversible loss of shape or dimension may also occur. Such is the case for materials having properties similar to a metal spring, in which case the deformation is totally reversible. As used herein, xe2x80x9ccreep resistantxe2x80x9d means that the material resists the tendency to creep, through for instance, chemical structure, physical structure, and the like.
To make such composite materials, at least one layer of a fibrous web is laminated to at least one facing layer. Lamination may occur, for instance, by passing the layers through the nip between two rolls, one roll being a calender roll, and the other roll being an anvil roll, to compression bond and laminate the layers together. The calender and/or anvil rolls have traditionally been patterned in some way, otherwise known as point bonding, so that the resulting laminate material was not bonded across its entire surface. As a specific example, a fibrous web of an elastic continuous filament and meltblown fiber has been point bonded to a facing layer while the continuous filament web was in a stretched state as described in commonly assigned European Patent Application 0 548 609 A1 to Wright. Upon release of the tension, the laminate would retract, thereby xe2x80x9cgathering upxe2x80x9d the facing layer. The two-fold advantage of this was that 1) a more xe2x80x9ccloth-likexe2x80x9d appearance resulted, and 2) the inelastic layer could return to its pre-gathered dimension, thereby capitalizing on the elasticity of the continuous filament web.
One disadvantage of this method of lamination, though, was that the patterned (e.g. Ramisch) rolls used in point bonding can damage the elastic filaments as can be seen, for instance, in FIG. 4. FIG. 4 is a scanning electron micrograph of an elastomeric continuous filament layer 118 which has been attached to an elastomeric meltblown fiber layer 126 and bonded using patterned rolls. Several of the continuous filament strands have been torn, nicked, cut and the like as exemplified by 118xe2x80x2. Such damage affects the elastic properties and, thus, the performance of the material and laminates thereof by causing the fibers to completely break during use, at body temperature, and under stretched conditions as can be seen in FIG. 5. FIG. 5 is a scanning electron micrograph of the material of FIG. 4 after being subjected to use conditions. If the filament is broken, then the inherent latency of that particular filament will have little or no affect on the material and thus will not contribute to the product conformability to the body.
Commonly assigned PCT publication number WO 98/29251 to Thomas et al., describes a means of solving this problem by utilizing two smooth calender rolls to bond the layers together. Smooth roll calendering resulted in improved dimensional stability (e.g., less stress relaxation and creep) because the continuous filaments were not broken during calendering as can be seen in FIG. 6. In FIG. 6, a material as described above for FIG. 4, has been bonded this time with the smooth calender rolls. The laminate exhibited improved inherent latency by not damaging the elastomeric continuous filaments. A disadvantage of this method was that the resulting material lost its loftiness (i.e., it was flat), thus having little or no xe2x80x9ccloth-likexe2x80x9d aesthetics. In FIG. 7, on the right side of the photo, a similarly made material has been smooth roll calendered as compared to the material on the left side of the photo, in which the material was patterned roll calendered The material made using smooth roll calendering is clearly less lofty, and therefore less attractive to the consumer.
A need, therefore, exists for a method of manufacturing a composite elastic material that is dimensionally stable by controlling inherent latency and also has xe2x80x9ccloth-likexe2x80x9d aesthetics. Additionally, there is a need for a method of manufacturing a composite elastic material without damaging the fibrous layers during the manufacturing process. The present invention avoids these and other difficulties by providing such a process and articles formed therefrom.
The present invention provides a method of producing a creep resistant composite elastic material wherein an elastic fibrous web is bonded to at least one gatherable layer to form a composite elastic material. By passing the composite elastic material into a nip formed between an anvil calender roller and a point un-bonded calender roller, wherein the point un-bonded calender roller has recessed areas in the surface of said roller, a material which is creep resistant, dimensionally stable, and has inherent latency, is formed.